Newaye Daniel was driving across Boston when his phone rang. Actually, "sang" might be a better description: the phone started playing the tune "U Don't Know Me" by Southern rapper T.I. The song is one of 11 ringtones--looped snippets of music that replace default rings--the 22-year-old has on his cell phone. Like so many of his peers, Daniel has assigned different rings to different people: when people he doesn't know dial, the phone rings with, well, "U Don't Know Me." If his mother calls from her home in Virginia, the Sarah McLachlan song "I Will Remember You" issues forth, usually from his hip pocket. "Sometimes the expressions on people's faces is to die for," he says. "Last night my girlfriend heard the Sarah McLachlan song when my mom called and she was all 'that's the cutest thing ever."

Since the late 1990s, ringtones have evolved from "monophonic," one-cheesy-note-at-a-time synthesized melodies, to more complex "polyphonic" renderings of pop songs (think Casio-keyboard cover versions). Users either select the songs from their phone's Web browser or subscribe to a service that lets them download sounds by keying the tunes' code numbers into their phone. Now it is even possible to download "master tones," actual clips of songs as you would hear them on the radio. With these technological developments, a new industry has been born: last year alone, U.S. cell-phone users spent $300 million on ringtones--a small slice of the estimated $4 billion worldwide ringtones market, which is huge in Europe and Asia. Lewis Ward, a senior analyst at IDC Research, projects that by 2008 Americans will spend $1.8 billion on ringtones.

Ringtones have become so big that last October Billboard magazine created a chart for them (the No. 1 ringtone for the last five weeks has been "Lovers and Friends" by the ubiquitous crunkster Lil Jon). "There's been so much attention on the music industry suffering woes, with three years of album sales declines in 2001, 2002, 2003, music didn't seem to be as big a deal as it used to be," says Geoff Mayfield, Billboard's director of charts. "Ringtones dispel the notion that music is no longer relevant to kids." While hit songs are rarely downloaded off of Internet services like Apple's iTunes more than 50,000 times, Mayfield says it is unusual for the top ringtone to sell fewer than 90,000 copies. With polyphonic tracks costing an average of $1.65 per ring and master tracks commanding $2.75, tacked on to the users' monthly phone bill, the music industry has taken notice.

With the trend in ringtones heading toward master tracks--real clips of real music--the record labels that own the hottest recordings are getting into the game. MTV is currently launching a new franchise called Made Hear, which will kick off with an actual album of ringtones created by hip-hop superproducer Timbaland. Through partnerships with major labels, the network offers subscribers exclusive ringtones from artists ranging from Green Day to Ryan Cabrera to 50 Cent.

"How would you like to be the first to have what was No. 1 on 'TRL' ['Total Request Live'] today?" asks MTV's Greg Clayman. "If you can sell a million ringtones and you can have them out on the streets as phones ring, it seems like a good way to promote." A company called BlingTones is attempting to become the first "wireless record label" by offering ringtone-exclusive tunes by well-known producers like Q-Tip, Denaun Porter and Rockwilder. "You can do a 30-second song, and it hits a whole different set of ears," says Rockwilder, who has laid down tracks for such rappers as Jay-Z, Busta Rhymes and Redman. "People who don't even buy records will definitely buy ringtones for their phone."

Zingy Inc., a company that sells some 3 million ringtones a month in the U.S., also offers original voice ringtones, which founder Fabrice Grinda says make up 10 percent of Zingy's sales. It's a clever way for artists to cut out the middleman. One features the voice of Ludacris, hip-hop's reigning clown prince, who shouts, "This is Ludacris right here; pick up your phone! Hurry it up, man, quit wasting time." Porn impresario Jenna Jameson has recorded the first "moan tones," the sounds of her voice as she presumably does what made her famous. The London Symphony Orchestra is even selling classical ringtones.

But wait, there's more. Last year T-Mobile launched the first stateside ringback service. Ringbacks are songs and sounds that can be downloaded in place of the rings that the caller hears. Instead of sitting through eight ring-ring-rings before a buddy's voicemail picks up, callers are treated (or subjected, depending on one's taste) to, say, J. Lo's newest ditty.

So what's the logic behind paying $3 for a fraction of a song that you can only hear when someone calls you? (After all, a fan can own a full song for a third as much--it costs just 99 cents to download a track from iTunes.) "If you're in high school and you have the latest Snoop Dogg or 50 Cent, it shows that you're in," explains Zingy's Grinda. "It's a means of expressing your coolness." It's also a means of self-expression, an age-old adolescent way of announcing to the world "this is who I am" every time the phone goes off in public.

For others, it's something more practical. "The rings that come with the phone usually are really annoying," explains Tufts University student Valerin Lopez, 21. Her own favorite ringtone is the theme song to the kitschy 1980s television show "Knight Rider." Still, Lopez unwittingly illustrates the classic conundrum of a ringtone believer: "It takes me a long time to answer the phone," she admits, "because I like listening to the song."